[Pw_forum] confused about nonzero total charge

[Paolo Giannozzi]

Fan Yang wrote:

>  I want to calculate the charge of atoms in cell, and I'm not sure 
> whether the process is correct. [...]
> Once I obtain the charge of every atom in unit cell, the total charge of 
> each atom does not equal 0, which confused me because the system of 
> total charge is set as 0.

As explained approximately 1001 times in this mailing list,
"the charge of an atom in a molecule/solid" typically depends on a
particular choice of atomic orbitals over which wavefunctions are
projected. The true total charge is zero, the charge you are
talking about is not. From code projwfc.f90 :

     psum = SUM(charges(:,:,:)) / nelec
     WRITE( stdout, '(5x,"Spilling Parameter: ",f8.4)') 1.0d0 - psum
     !
     ! Sanchez-Portal et al., Sol. State Commun.  95, 685 (1995).
     ! The spilling parameter measures the ability of the basis provided
     ! by the pseudo-atomic wfc to represent the PW eigenstates,
     ! by measuring how much of the subspace of the Hamiltonian
     ! eigenstates falls outside the subspace spanned by the atomic basis

Paolo
-- 
Paolo Giannozzi, Democritos and University of Udine, Italy